/*
  @ 名称: 解决 <select /> z-index 太高问题
  @ 描述: 浮层被 select 穿透，是最常见的 z-index bug
  @ 例子：
     <div id="dd3" class="sl-selectmask" style="background:#FF3366;">
      内容<br/>
    <!--[if lte IE 6.5]><iframe src="javascript:'';"></iframe><![endif]-->
    </div>
*/

.sl-selectmask {
  position: absolute;
  left: 0;
  top: 0;
  z-index: 10;
  overflow: hidden;
  width: 33em;

  iframe {
    position: absolute;
    top: 0; 
    left: 0; 
    z-index: -1;
    border: none;
    filter: mask();
    @include square(3000px); /* for any big value */
  }
}